Larger than recommended max HP

[jkimball]

Just curious, on a boat such as mine which has a dry weight of 1790 lbs. ('99 FW Horizon 170) but the recommended max HP is 115, what would be the consequences or drawbacks of repowering with a larger motor? Say... 150-175?

The reason I ask is because, from what I've been told is that a lot of the reason to stay with the max rated HP is due to too much weight being hung off the transom and trim issues due to the weight. But, I challenge this because a lot of the smaller drag boats such as Hydra-sports Vipers etc. have extremely lightweight hulls and run high HP motors. Example, there is a guy that regularly brings one out on the creek here that is a '77 with a Merc. 2.5 on it and the dry weight of his hull is only 450 lbs. Granted, his boat has a front mounted fuel tank to kind of counter the weight.

And I understand that my boat isn't "Drag Boat" nor is it specifically designed for a high HP motor, but my situation wouldn't be quite as extreme as this as my hull weighs almost 4x his and I wouldn't be looking to put a 2.5 on mine either...(though... that could be fun :eyebrows

But, with the weight of my boat and looking to put a 150-175 if and when I do repower (my 115 runs like a top right now) would there be any adverse effects?

 

[Slip Away]

Re: Larger than recommended max HP

Adverse effect could be ripping the transom off the boat and sinking. That's all I can think of at the moment.

 

[MTboatguy]

Re: Larger than recommended max HP

For one, you would be breaking regulations and could be ticketed if law enforcement decided to inspect your boat. The second thing to worry about, would your transom hold up to the extra weight and horsepower, A 15 year old boat could have stress weakening in the transom, that could end up dropping the motor in the drink and breaking the transom, which of course would put you and any passengers in danger, of course adding weight, will decrease carry capacity, equaling less people and gear on the boat. I have over powered in the past, but never by more than 10% rating. Remember, there is a reason it is rated as it is, changing the weight distribution on the boat could cause poor handling as well.

If you want more power, the best suggestion is sell your boat and buy one that is rated for the type of power you want.

You need to remember those drag boats are built to handle that type/size of engine structurally, if your was, it would be rated higher.

 

[garbageguy]

Re: Larger than recommended max HP

It's one thing if you modify your boat outside recommendations of those who design the entire boat, and you run the boat yourself - it's quite another if you take someone with you, or are in waters where there are other boats

 

[stackz]

Re: Larger than recommended max HP

I say mainly safety sake.

have some friends with smaller boston whaler and I simply wont ride on the boat with them. yeah it dips down a little in the rear when not under power but there's no reason for a 17' whaler to be able to do 60mph. even when the water is glass, that boat is squirrelly at best.

 

[jkimball]

Re: Larger than recommended max HP

These are all things that I was thinking would be issues, I just wasn't sure. Appreciate the help fellas

 

[Mel Taylor]

Re: Larger than recommended max HP

I can't do much to answer your question, but I will give what information I do have. Years ago an acquaintance of mine owned a boat somewhere in the 16 ft range. I no longer remember make or model, but it was an open semi-vee hull fishing boat. He also had a lot more money than brains (IMO) and more cojones than I've got. He started out with a 35 horse motor (probably close to what the boat was rated for), then began to amp it up with bigger motors in some thing like 10hp increments. The last I remember hearing, I think he had a 75 horse motor on it and it worked fine except for one minor problem. the faster he went the more unstable the steering got and at WOT the boat was almost unmanageable. I believe he had some degree of steering problem with every motor he tried that was much over the rated hp for the boat. I never did know what the end result of his experimentation was but as of last year he was still alive so I assume he eventually went back to a more conventional sized motor.

 

[Starcraft5834]

Re: Larger than recommended max HP

adverse effect could be ripping the transom off the boat and sinking. That's all i can think of at the moment.

lol!!!!!!!!!!!!!

 

[briangcc]

Re: Larger than recommended max HP

It'll sit lower in the water for one due to the increased weight. And that extra weight may also affect the handling of the boat. It'll also change the draft of your boat as its now lower in the water so items you might have just cleared with your previous engine are now struck with the new one.

When I went from a 2 Stroke 120HP Force to a 4 Stroke 115HP Honda on my Bayliner, it sat a good 1-2" lower in the water. I know once I repowered, if I had someone on either of the back platforms I would have water coming in the splash well drains as it sat that low in the water. The reason being is that my Bayliner wasn't designed to handle the extra 100 or so lbs of 4 stroke sitting on the transom. I didn't have this issue when I had the lighter Force engine - I had other issues though with that POS engine which is why I repowered with Honda!


I guess the better question is...what problem are you trying to fix with a larger engine? Are you suddenly slower then when you bought the boat?

 

[Scott Danforth]

Re: Larger than recommended max HP

three things come to mind

drag boats are purpose built hulls with heavily reinforced motor plates to take the big motor on a small hull. There are no rules, and there is high premium racing insurance to cover things when the boat destroys itself and sometimes the driver

your boat is not a drag boat.

It is illegal to exceed the rating hp on the hull.

 

[Home Cookin']

Re: Larger than recommended max HP

Hull weight is probably not the comparative factor in this analysis and probably has very little relevance, due to the hydrodynamics of the hull on a plane being the most important factor. Or aerodynamics once they really get dancing. You aren't pushing square-front barges here.

In fact--and I'm just guessing--a light weight hull might handle a bigger motor with a comparatively weaker transom, b/c the light boat (and its hydrodynamics) present less resistance that the motor is pushing against. Think of it this way--there is more stress on your transom if you are pushing another boat than when you are just driving yours.

 

[jkimball]

Re: Larger than recommended max HP

All great points guys, and I'm not saying that I want to put a larger motor on her, but I was just curious of what the "real world" consequences would be of doing it.
In all reality, if/when I do repower... if I don't end up upgrading to a different boat I would really like to put an E-tec 115 HO.

The little 115 I have has plenty of power for the boat, and with my usual load (2 adults & 3 children) it pushes it just fine and doesn't really have any issues getting on or holding plane. My WOT is about 42-43 MPH at 5100 and I plane at 2700. It would sometimes be nice to have a little more power when loaded with all adults and trying to get on Plane, but for now it's just as simple as changing the prop and trimming her in.

Again, Not saying I want to overpower... just curious.

 

[ondarvr]

Re: Larger than recommended max HP

It is only illegal in some locations, it's not a federal regulation. So depending on where you live it may be OK from that angle.

I'm not recommending it though.

 

[MTboatguy]

Re: Larger than recommended max HP

It is only illegal in some locations, it's not a federal regulation. So depending on where you live it may OK from that angle.

I'm not recommending it though.

The rating plate on the boats is a "US Coast Guard" document and the Coast Guard actually shows up where you least expect them, I was stopped a few years ago on Soap Lake, WA by a US Coast Guard Agent who inspected my boat. Remember those tags specifically say:

"This Boat Complies with US Coast Guard Safety Standards in Effect on the Date of Certification"

 

[dazk14]

Re: Larger than recommended max HP

All great points guys, and I'm not saying that I want to put a larger motor on her, but I was just curious of what the "real world" consequences would be of doing it.
In all reality, if/when I do repower... if I don't end up upgrading to a different boat I would really like to put an E-tec 115 HO.

The little 115 I have has plenty of power for the boat, and with my usual load (2 adults & 3 children) it pushes it just fine and doesn't really have any issues getting on or holding plane. My WOT is about 42-43 MPH at 5100 and I plane at 2700. It would sometimes be nice to have a little more power when loaded with all adults and trying to get on Plane, but for now it's just as simple as changing the prop and trimming her in.

Again, Not saying I want to overpower... just curious.

Well, that's easy. If you're running a 3 blade, Just switch to a 4 blade stainless and raise up the rpms a bit lightly loaded. $275. It'll have a whole new lease on life and pull like a freight train.

Just so you are aware. Bigger motors don't always accelerate faster. They have to be propped bigger so they don't over-rev and that sucks life out of the hole shot.

 

[jkimball]

Re: Larger than recommended max HP

Well, that's easy. If you're running a 3 blade, Just switch to a 4 blade stainless and raise up the rpms a bit lightly loaded. $275. It'll have a whole new lease on life and pull like a freight train.

Just so you are aware. Bigger motors don't always accelerate faster. They have to be propped bigger so they don't over-rev and that sucks life out of the hole shot.

Not trying to turn this into a prop topic and have the post moved but;

I am running a 3 blade now, both are stainless. I have a 17P Stainless OMC SST & a 19P Stainless Viper, both of which have proven to be great props, especially compared to the banged up 19P Aluminum that was on it when I got it.

However, the 19P Viper seems a little more sluggish out of the hole than the old 19P Aluminum did... BUT I get about 300 more RPMs at WOT than I did with the Aluminum. Maybe the sluggish start could be due to a heavier rotating mass at lower RPMs?

Now you say, go with a stainless 4 Blade (which I've really been considering but don't know what pitch) and raise RPMs a little. Did you also mean to drop pitch in order to do this?

Even though I don't run WOT all the time, I don't want to carve off too much of my top speed due to the extra blade, but it seems the benefits of a 4 blade outweigh the costs. And would there be much reason to not return to an Aluminum 4 blade?

In my experience it seems that Aluminum spools up faster but Stainless offers less slip and higher WOT RPMs... does this sound about right?

 

[dazk14]

Re: Larger than recommended max HP

Well, a properly fitted stainless will usually accelerate quicker, run faster, but may have additional cupping and more progressive pitch that will pull down the WOT rpm's.

When the original SST came out in the early70's, that was the only choice. Today there are countless. The viper is fast, but it does have more diameter than I think a 17'er needs.

I'd suggest, posting your speed, rpm, engine, wot range and gear ratio as a prop topic. The rpm range and gear ratio we can pull by simply supplying the specific 115 you have and decipher how it's responding to the different props you have.

You'll likely be able to raise the motor a couple holes to improve acceleration and top speed. No question stainless as far as materials.

Just to keep this a non-prop topic.

As was said before, besides ripping the transom off the back of the boat with a V-6, I see nothing wrong with it. As it sits, you likely have a 115 prop shaft rated motor, that would have been called closer to a 135hp in older years.

If you have a good running motor, for the cost. I think an etec wouldn't give you much bang for the buck. If you have a need for speed, go pickup a $4k used Yamaha GP1300R 2 up waverunner, that'll run 65mph with arm ripping acceleration.

 

[flipbro]

Re: Larger than recommended max HP

Back when i was younger(And didnt Know better) i reinforced my transom with alluminum plate and dropped a 125 hp merc tower of power with a 2 blade bronze prop on the back of my 15.5 foot peterbourgh it was rated for only 85 hp. I run that boat 3 seasons then sold it to a friend that run it another 2 seasons before he pulled the engine off and put on an old Jcraft boy it was fast. But looking back likly not the smartest thing we ever did!!!!

 

[oldandintheway]

Re: Larger than recommended max HP

It would probably void your insurance coverage.

 

[thumpar]

Re: Larger than recommended max HP

It would probably void your insurance coverage.

I was thinking the same thing. If something were to happen the insurance company could decline a claim because of the over rated power.